From the update changelog:

We made major performance optimizations targeting mid-to-late game colonies. On an example machine, we saw 20-30% additional TPS in late game colonies. We rewrote many systems to spread out their workload. The pathfinding system is now fully multithreaded and batched, and the lighting system is also multithreaded. Pawns/Things are now ticked less often in certain circumstances (e.g. when off camera). There are also several optimizations to minor systems and fixes to major memory leaks.

We significantly reduced game launch times in version 1.6! For example, on our development machines version 1.5 previously launched in 18.57s with all expansions. Now version 1.6 launches in 4.66s - and that's with an additional expansion!